The present invention relates to a consumable electrode pulse arc welding machine for performing welding by shielding a weld part with a gas containing an inert gas as a main component and continuously feeding a consumable electrode wire (hereinafter referred to simply as an "electrode wire") to the weld part, in which welding voltage control is applied to the welding machine while the electrode wire is transferred to the weld part in the form of spray by using a pulsating welding current (hereinafter referred to simply as "the pulse MIG welding method").
The conventional pulse MIG welding method uses a pulse current so that the molten wire metal is transferred to a base metal positively in the state of spray, and hence it has various different characteristics as compared with what is called the normal MIG welding method using a constant DC current. One of the great differences is that, when the welding output current is low, in the normal MIG welding method, the short-circuiting and the arc generation are alternated to effect welding, while, in the pulse MIG welding method, the molten wire metal may be transferred to the base metal in the form of spray. The spray transfer causes a great difference in a spatter generated as compared with the short-circuiting transfer. That is, the fact that the transfer in the form of spray greatly reduces the generation of a spatter as compared with the normal short-circuiting transfer arc welding is one of remarkable features of the MIG welding method. In the pulse MIG welding method, short-circuiting occurs between the electrode wire and the base metal when an arc voltage is set at a lower level to decrease the arc length. However, if the arc voltage is set to be too low in this case, short-circuiting occurs more frequently and this undesirably increases the generation of a spatter to the same degree as the normal MIG welding method. On the other hand, if the arc voltage is set at a higher level to increase the arc length, the occurrence of short-circuiting between the electrode wire and base metal is reduced, however, the arc is likely to become unstable, with the result that no arc occurs at a desired position, thereby making satisfactory welding impossible.
FIG. 1 is a graph showing the relationship between the welding arc voltage and the frequency of short-circuiting when the wire is fed at a rate of 6.3 m/min (the wire diameter is 1.2 mm.phi.). It is seen that the frequency of short-circuiting is increased greatly as the arc voltage decreases; while no short-circuit occurs at the arc voltage of 24 V or higher.
FIG. 2 is a graph showing the relationship between the welding arc voltage and the amount of a spatter generated under the same welding condition as in the case of FIG. 1. It is seen that the generation of a spatter increases with a decrease in the welding arc voltage, that is, with an increase in the frequency of short-circuiting. At the welding arc voltage of 24 V or higher, no short-circuiting occurs and almost no spatter is generated. The detailed experimental results show that when the welding arc voltage is raised to 24 V or higher, undesirable welding defects such as undercut appear with an increase in the welding speed. When the welding arc voltage is at a lower level, on the other hand, any undercut does not occur even in the case of high speed welding, but the generation of a spatter increases undesirably. It has thus been found that, when the arc voltage is in the range of 23.+-.0.5 V, as shown in FIG. 1, the number of times of short-circuiting is not more than 20 per sec and the amount of a spatter generated is small, thereby making it possible to effect the most satisfactory welding. The arc length involved in this case is about 4 mm.
As described above, the conditions under which the pulse MIG welding method shows its features are quite limited. As seen from the example illustrated in FIGS. 1 and 2, the welding arc voltage has a tolerance of only about one volt. Further, the adequate voltage value depends greatly on the welding position, joint form, kind of a gas and other welding work conditions. With regard to the joint form, for example, an optimum voltage value for the fillet welding is lower by about 0.5 V than that for the flat butt welding.
As described above, the problems in the pulse MIG welding method reside not only in the fact that the conditions for an adequate voltage are limited but also in the fact that the adequate welding conditions depend on the welding work conditions. As a result, there has been great difficulty in performing the welding work, and it has been a great problem in the field welding operation.